Video data is encoded in a highly efficient encoding scheme when it is transmitted from a transmitter device to a receiver device, when it is stored in a storage device, or in other situations. “Highly efficient encoding” is an encoding process to transform a data stream into a different data stream in order to compress the data volume.
As a highly efficient encoding method for video data, an intra-picture prediction (intra prediction) encoding method is known. This encoding scheme utilizes high spatial correlation in video data. That is to say, in this encoding scheme, it is possible to restore an image in a picture by using only information included in that picture without using other pictures. As another highly efficient encoding method, an inter-picture prediction (inter prediction) encoding scheme is known. This encoding scheme utilizes a characteristic in which video data has a high correlation in the time direction. Note that a picture at a certain timing is similar to a picture at an immediately previous timing in video data. An inter-prediction encoding scheme utilizes this characteristic so as to encode pictures.
In the inter-picture prediction encoding scheme, a current picture is divided into a plurality of blocks. An encoder selects, for each block, a region similar to the current picture block from the decoded picture of an encoded frame, and calculates the difference between the selected similar region and the current picture block so as to remove redundancy in the time direction. Then, vector information representing the similar region and the difference information with the redundancy having been removed are encoded so as to achieve a high compression rate.
For example, in a data transmission system employing inter-prediction encoding, a transmitter device generates motion vector data representing a “motion” toward a target picture from the previous picture, and also generates difference data between the predicted image of the target picture and the actual image of the target picture, and transmits to the receiver device the generated motion vector data and difference data. The predicted image is generated using the previous picture and the motion vector data. The receiver device reproduces the target picture from the received motion vector data and difference data. Note that as moving picture encoding schemes, there are ISO/IEC MPEG-2 and MPEG-4 (hereinafter, referred to as MPEG-2 and MPEG-4).
According to MPEG-2, I-picture, P-picture, and B-picture are defined. The I-picture is obtained by encoding information only within the picture. The P-picture is obtained by performing forward-direction inter-picture prediction in accordance with a past picture and encoding the prediction error. The B-picture is obtained by performing bi-directional inter-picture prediction in accordance with a past picture and a future picture, and encoding the prediction error.
Highly efficient methods (i.e., a high compression rate) are desirable in video encoding/decoding. As a method for enhancing encoding/decoding efficiency, a signal encoding method including the first through third steps described below is proposed. The first step obtains as a reference signal a signal in a correlation with an encoding target signal. The second step derives, in accordance with the characteristic of the obtained reference signal, a transform basis serving as fundamentals of the transform rule. The third step transforms the encoding target signal in accordance with the transform rule in accordance with the derived transform basis so as to encode the encoding target signal. (Japanese Laid-open Patent Publication No. 2002-314428 for example).
Also, as a related technique, a predictive decoding device is proposed in Japanese Laid-open Patent Publication No. 2000-59785.
As described above, highly efficient methods are desirable for video encoding and decoding. Further, various methods for enhancing efficiency in encoding/decoding are proposed. However, further improvement is required in encoding and decoding of video data.
When, for example, image information is to be encoded, an orthogonal transform or the like may be performed in order to generate a coefficient sequence representing that image information. The generated coefficient sequence is scanned using a specified pattern, and thereafter entropy encoded. However, according to conventional techniques, the encoding/decoding efficiency becomes lower depending on characteristics of images.